1. Field of the Invention
The invention relates to device for operating a dielectric barrier discharge lamp. More particularly, the invention relates to an image scanning device with a light source that is a fluorescent lamp using a dielectric barrier discharge.
2. Description of the Prior Art
As an example of technical literature relating to dielectric-barrier discharge lamp, JP patent disclosure document HEI 2-7353 (U.S. Pat. No. 4,983,881) discloses a radiator device, i.e. a dielectric barrier discharge lamp in which a discharge vessel is filled with a discharge gas and in which a dielectric barrier discharge forms excimer molecules, from which light is emitted. This dielectric barrier discharge is also called an ozone production discharge or a silent discharge, as is described in the Discharge Handbook, Denki Gakkai, June 1989, 7th edition, page 263.
Since this dielectric barrier discharge lamp has various features which a conventional low pressure mercury lamp or a conventional high pressure arc discharge lamp does not have, there are diverse possible applications. Especially in view of the great interest in the problem of photochemical environmental pollution, recently much interest has been shown in this photochemical reaction by UV radiation.
FIG. 1 schematically shows an arrangement which explains the working principle of a dielectric barrier discharge lamp. In the figure, reference number 2 indicates a dielectric barrier discharge lamp having one dielectric or two dielectrics (6,7) between electrodes (4, 5) and which surround a discharge plasma space (3). In FIG. 1, a lamp bulb (8) functions as the dielectric (6, 7).
In the operation of the dielectric barrier discharge lamp 2, an ac voltage of 2 kV to 10 kV with a high frequency of, for example, 10 kHz to 200 kHz is applied to the electrodes (4, 5) at its two poles by a feed device 1 and an associated feed line 11. Due to the dielectrics (6,7) between the discharge plasma space 3 and the electrodes (4, 5), current does not flow from the electrodes (4, 5) directly into the discharge plasma space 3, but rather the current flows by means of the action of the dielectrics (6,7) which acts as a capacitor. This means that, on the surfaces of the dielectrics (6,7) and on the side of the discharge plasma space 3, an equivalent electrical charge is induced by polarization of the dielectric by the respective electrode (4,5). However, the induced electrical charge on the dielectrics has the opposite polarity than that of the electrodes. Between the surfaces of the dielectrics (6,7), which are located opposite to one another and surrounding the discharge plasma space 3, a discharge takes place.
Since only little current flows along the surfaces of the dielectrics (6, 7) on the side of the discharge plasma space 3 where the discharge takes place, the electrical charge induced on the surfaces of the dielectric (6,7) on the side of the discharge plasma space 3 is neutralized by the electrical charge moving through the discharge. Therefore, the electrical field of the discharge plasma space 3 is reduced. The discharge current soon stops even if the application of the voltage to the electrodes (4, 5) is continued to be applied. However, in the case in which the voltage applied to the electrodes (4, 5) continues and increases, the discharge current remains uninterrupted.
In the case in which the discharge stops after a discharge has occurred once, a discharge does not occur again until the polarity of the voltage applied to the electrodes (4, 5) is reversed.
In the case of, for example, a dielectric barrier discharge lamp is filled with xenon gas, the xenon gas is split by the discharge into ions and electrons, yielding a xenon plasma. In this xenon plasma, when excited to a certain energy level, forms excimer molecules. The xenon excimers dissociate after a certain lifetime, and the energy released in the process is emitted in the form of photons with vacuum UV wavelengths. It is desirable to form these excimer molecules with high efficiency so that the dielectric barrier discharge lamp can be operated as vacuum UV light source with high efficiency.
The greatest obstacle to efficient formation of excimer molecules during discharge is the excitation of the discharge plasma to energy levels that do not contribute to the formation of excimer molecules.
Electron movement of the discharge plasma immediately after starting the discharge takes place in groups when the energy is high but the temperature is low. In this state there is a great probability that the discharge plasma transitions into the resonant state that is necessary for formation of the excimer molecules. When the discharge interval is prolonged, electron movement of the plasma however gradually transitions into a thermal state, i.e. into the state of thermal equilibrium called the xe2x80x9cMaxwell-Boltzman distributionxe2x80x9d. Thus, the plasma temperature rises and the probability of transition into a more highly excited state becomes greater where excimer molecules cannot be formed.
Moreover, sometimes when excimer molecules have been formed, a subsequent discharge will break down the excimer molecules before their lifespan elapses and they decompose naturally by emitting the desired photon. In fact, in the case of xenon excimers, a period of about 1 microsecond is necessary between the beginning of discharge and emission of a vacuum ultraviolet photon, and a subsequent discharge and repeated discharge in this time interval reduce the efficiency of the excimer emission. Therefore, it becomes apparent that it is most important to reduce the energy of the subsequent discharge as much as possible once the dielectric barrier discharge lamp has started.
The voltage applied to the dielectric barrier discharge lamp is not a sinusoidal voltage. However, a voltage which has a steep change is suitable. To improve the efficiency of the dielectric barrier discharge lamp, based on these understandings, the technique in which a more or less rectangular voltage waveform is applied is described, for example, in Japanese patent disclosure document HEI 11-317203 (U.S. patent application Ser. No. 09/555,512, now U.S. Pat. No. 6,369,519). This publication discloses that together with applying a voltage with a steep rise the time for suppressing a subsequent ringing is shortened.
This technique meets the above described precondition in which it is important to reduce the energy of the subsequent discharge as much as possible once the dielectric barrier discharge is started so as to achieve an outstanding effect with respect to efficiency of the formation of excimer molecules.
The above described dielectric barrier discharge lamp is an outstanding arrangement with respect to efficiency. However, it has been considered to be disadvantageous in that the irradiance of the dielectric barrier discharge lamp drops over the course of use.
Especially in the case of using this dielectric barrier discharge lamp as a fluorescent light source of an image scanning device, the sensitivity of the CCD sensor as an image scanning element drops over time when the irradiance decreases. This results in the extremely serious defect in which imaging using the dielectric barrier discharge lamp as a light source is not possible. Furthermore, the phenomenon in which the irradiance decreases over the course of use is undesirable, not only in a light source for an image scanning device but in similar devices that require constant light level output. Besides the conventional technical object of increasing the efficiency, there is therefore a great demand for achieving the new object of maintaining the irradiance, i.e. preventing a reduction in irradiance. The technique in which a dielectric barrier discharge lamp is used as a light source of the above described image scanning device is described in commonly-owned, co-pending U.S. patent application Ser. No. 10/014,453 which is hereby incorporated by reference.
An object of the present invention is to devise a device for operating a dielectric barrier discharge lamp in which not only the efficiency is maintained, but in which a reduction of the irradiance over the course of use is advantageously prevented.
A device for operating a dielectric barrier discharge lamp of the present invention comprises a dielectric barrier discharge lamp and a feed device for applying a high voltage to this dielectric barrier discharge lamp, wherein this feed device via a set-up transformer applies a high voltage with an essentially periodic waveform to the above described barrier discharge lamp. Further, when the voltage polarity changes for the subsequent dielectric barrier discharge after completion of a preceding dielectric barrier discharge, the waveform of this applied voltage first produces a steep rising waveform and, in the subsequent ringing, the ratio of the difference between a second extreme value point and a third extreme value point to the difference between the first extreme value point and a second extreme value point is less than or equal to 30%.
The object of the invention is achieved is in the arrangement illustratively shown in FIG. 2 wherein the above described ringing is essentially not present. After thoughtful consideration of the above described object of the invention, it was found that the reason for the reduction of the irradiance of a dielectric barrier discharge lamp (hereinafter also called only a xe2x80x9cdischarge lampxe2x80x9d) over the course of use is the temperature increase of the discharge gas. This temperature increase is formed by a periodic voltage being continuously applied, and, as a result, it becomes accumulative. It is believed that due to this temperature increase, the formation of excimer molecules within the discharge vessel does not take place with high efficiency. Furthermore, this temperature increase is caused by an oscillation voltage waveform which forms after applying the voltage with a steeply rising waveform to the discharge lamp. The time determination of the oscillation disclosed in the above described Japanese patent disclosure document 11-317203 in and of itself is not sufficient, but it was found that the magnitude of the oscillation in itself (i.e., amplitude) must be fixed. Specifically, it is effective to prevent a reduction of the irradiance in an oscillating voltage waveform that is formed after applying the voltage waveform with a steep rise for producing a dielectric barrier discharge. It is preferable to make this amplitude as small as possible, so as to not allow the ringing or oscillation to arise.
The invention is described below using several embodiments shown in the drawings.